More and more industrial processes either are benefiting, or will be able to benefit further by using or introducing high power microwave technology into their manufacturing processes. Much of the benefit will come in the pre-cooked food processing area, as well as processes such as microwave sterilization and/or pasteurization, de-watering, heating, blanching and curing. Traditionally, high power microwaves are used to either soften hard-frozen blocks of food such as meat products in order to allow them to be sectioned and processed prior to either sale as frozen food products, or further processed. Other industrial microwave applications actually involve cooking the product prior to sale.
These food products range from pre-cooked bacon, meat and poultry products. Other food products include vegetables such as potatoes and beans in many varieties of process configurations. In addition to industrial food processes, there are also many, many non-food industrial microwave applications including building materials manufacturing like laminated veneer lumber and plywood.
In all of these applications, a means must be provided that allows the high power microwaves to be applied to the product. Traditionally, this was done by simply conveying the high power microwaves in a conduit such as waveguide from the high power microwave generator or transmitter to the microwave cavity where the products are exposed to the power flux and processed. This technology first saw wide use back in the late 1960's and 1970's. Then, the microwaves were introduced into a large open cavity where the inside physical dimensions of the microwave cavity were several times larger than the wavelength of the microwaves being used. This was done by design, in order to allow the microwaves inside of the cavity, (where the food or other products were usually conveyed inside the cavity volume by a continuous conveyor belt, or simply placed there in a batch process configuration).
The microwaves were introduced into the cavity, in most cases, by a simple, open-ended waveguide section, and allowed to “bounce” around inside of the microwave cavity. In this way, the process substrate inside the cavity would “swerve into” the high-energy microwave fields and be heated or otherwise processed. Specific systems of propagating microwave electric and magnetic fields are called “Modes”. Depending on many factors inside, especially the physical size of the cavity, these microwave modes can take on a variety of shapes and configurations. The greater the number of modes, the higher the statistical likelihood that the process product inside of the microwave cavity would encounter the microwave fields and be cooked or otherwise processed. As the number of microwave field configurations was increased, the probability of achieving a satisfactory process result was increased also. In the early days in order to get these microwaves bouncing all over the place, Raytheon developed a motorized wave guide antenna almost like radar. They actually called it a radar ring. It had a gear motor that turned the radar ring around at about one revolution a second, and it sprayed the microwaves down onto the food or into the cavity very much like a shower. Essentially, they had a microwave shower nozzle that physically rotated. The result was that when the food was going through sometimes it got sprayed and sometimes it didn't.
The physical dimensions of a microwave cavity as compared to the frequency, and therefore the wavelength of the microwaves, is the major determinant in how many of these different modes will be able to exist in the interior of a specific cavity's volume. In nearly all traditional industrial systems, the microwaves were simply “sprayed” into the cavity by an open-ended section of rectangular waveguide, and allowed to bounce around inside. The goal was to introduce the microwaves into the cavity so that a maximum number of microwave modes would be excited. In order to have the best chance of exciting the maximum number of microwave modes inside of the cavity, the microwaves were usually introduced through a rotating flat disk, upon which were usually three open-ended waveguide sections, set at approximately 120 degree angular displacements around the disk's edge, and then fed from the center. The disk was connected to a gear motor and physically rotated inside of the cavity. (The goal is the same as that accomplished by the turntable inside of a home microwave oven.)
This design approach worked, however, there are many problems associated with this feed configuration. First, as time passed and the technology became more sophisticated and the microwave power levels continued to increase, the motorized rotary feed system became increasingly vulnerable to high power microwave burn-outs due to the ever-increasing microwave power levels. Secondly, many industrial microwave processes involve the generation of cooking by-products such as grease or fat from the process. This was a continuous problem because it would usually accumulate over time inside of the rotating components of the rotary feed, heating up in the high power microwave fields and eventually burn out, often destroying the rotary feed network.
Thirdly, the gear motor's rotation speed was quite slow, and the number of possible microwave mode events inside of the cavity during the time the food or other process products were inside to be cooked or processed was correspondingly slow as well.
This would oftentimes lead to inconsistent and sometimes unpredictable process results.